Catalytically active oxide materials which comprise more than one metallic element, in particular more than one transition metal, are particularly frequently used. In this case, the term multimetal oxide materials is used. Usually, multi-element oxide materials are not simple physical mixtures of oxides of the elemental constituents but heterogeneous mixtures of complex polycompounds of these elements.
As a rule, such catalytic gas-phase oxidations are carried out on a large industrial scale in fixed-bed reactors, ie. the reaction gas mixture flows through a fixed catalyst bed and the oxidative chemical reaction takes place during the residence time therein.
Most catalytic gas-phase oxidations are highly exothermic and are therefore advantageously carried out in practice in multiple contact tube fixed-bed reactors. The contact tube length is usually a few meters and the internal diameter of the contact tube is usually a few centimeters. Heat exchange media flowing around the contact tubes remove the process heat (cf. for example DE-A 44 31 957 and DE-A 44 31 949).
Fixed beds comprising finely divided, pulverulent, catalytically active oxide material are not very suitable for carrying out catalytic gas-phase oxidations since they are not usually capable of withstanding industrial loading with starting reaction gas mixture without hydraulic transport.
This means that the catalytically active oxide material is usually converted into moldings whose length is tailored to the internal diameter of the contact tube and is as a rule a few millimeters.
U.S. Pat. No. 4 366 093 generally recommends hollow cylinders (rings) as the preferred geometry of such moldings. The height and external diameter should be from 3 to 6 mm and the wall thickness from 1 to 1.5 mm. The only shaping methods considered by U.S. Pat. No. 4 366 093 are pelletizing or extrusion to give unsupported catalysts (the total hollow cylinder consists of catalytically active material which may be diluted with finely divided, inert material) or impregnation of carrier rings to give support catalysts. The disadvantage of annular unsupported catalysts having a wall thickness of .ltoreq.1.5 mm is that the mechanical stability during introduction into the contact tube is not completely satisfactory. The disadvantage of supported catalysts is that they are limited to those oxidic active materials which can be formed from solutions. In addition, a single impregnation results only in slight absorption of active materials.
U.S. Pat. No. 4 438 217 and U.S. Pat. No. 4 522 671 recommend unsupported catalyst rings which have an external diameter of from 3 to 10 mm, an internal diameter which is from 0.1 to 0.7 times the external diameter and a height which is from 0.5 to 2 times the external diameter and are based on multimetal oxides containing molybdenum as the main component, for the preparation of acrolein or methacrolein by gas-phase catalytic oxidation. With a view to the required mechanical stability, 1 mm is considered to be just possible as a lower limit of the wall thickness. However, the disadvantage of larger wall thicknesses is that they are associated with an increase in the diffusion distance out of the reaction zone, which promotes undesirable secondary reactions and hence reduces the selectivity with respect to the desired product.
U.S. Pat. No. 4 537 874 likewise recommends catalyst beds comprising unsupported catalyst rings based on multimetal oxides containing molybdenum as the main component for the preparation of .alpha.,.beta.-monoethylenically unsaturated aldehydes by gas-phase catalytic oxidation. The wall thickness of the hollow cylinders is 2 mm in all examples.
Annular coated catalysts help to resolve the contradiction which exists in the case of unsupported catalyst rings between required mechanical stability (increasing wall thickness) on the one hand and limitation of the diffusion distance out of the reaction zone (decreasing wall thickness) on the other hand, while maintaining the otherwise particularly advantageous ring geometry. The mechanical stability is ensured by the annular carrier, and the catalytically active oxide material can be applied in the desired layer thickness to the ring surface.
However, a very general problem in the case of coated catalysts is their production on an industrial scale, ie. they are to be prepared on an industrial scale in such a way that
they have the layer thickness required with regard to the catalyst activity, PA1 the catalytically active coat adheres satisfactorily in the required thickness to the surface of the carrier, PA1 the coat thickness shows very slight fluctuations over the surface of a carrier, PA1 the coat thickness shows very slight fluctuations over the surface of different carriers, PA1 the size of the specific, catalytically active surface area based on the mass unit of the active material is satisfactory and PA1 the output of the production process is satisfactory. PA1 X.sup.1 is W, Nb, Ta, Cr and/or Ce, PA1 X.sup.2 is Cu, Ni, Co, Fe, Mn and/or Zn, PA1 X.sup.3 is Sb and/or Bi, PA1 X.sup.4 is at least one or more alkali metals, PA1 X.sup.5 is at least one or more alkaline earth metals, PA1 X.sup.6 is Si, Al, Ti and/or Zr, PA1 a is from 1 to 6, PA1 b is from 0.2 to 4, PA1 c is from 0.5 to 18, PA1 d is from 0 to 40, PA1 e is from 0 to 2, PA1 f is from 0 to 4, PA1 g is from 0 to 40 and PA1 n is a number which is determined by the valency and frequency of the elements differing from oxygen in I. PA1 X.sup.1 is W, Nb and/or Cr, PA1 X.sup.2 is Cu, Ni, Co and/or Fe, PA1 X.sup.3 is Sb, PA1 X.sup.4 is Na and/or K, PA1 X.sup.5 is Ca, Sr and/or Ba, PA1 X.sup.6 is Si, Al and/or Ti, PA1 a is from 2.5 to 5, PA1 b is from 0.5 to 2, PA1 c is from 0.5 to 3, PA1 d is from 0 to 2, PA1 e is from 0 to 0.2, PA1 f is from 0 to 1, PA1 g is from 0 to 15 and PA1 n is a number which is determined by the valency and frequency of the elements differing from oxygen in I. PA1 X.sup.1 is W and/or Nb, PA1 X.sup.2 is Cu and/or Ni, PA1 X.sup.5 is Ca and/or Sr, PA1 X.sup.6 is Si and/or Al, PA1 a is from 3 to 4.5, PA1 b is from 1 to 1.5, PA1 c is from 0.75 to 2.5, PA1 f is from 0 to 0.5, PA1 g is from 0 to 8 and PA1 n is a number which is determined by the valency and frequency of the elements differing from oxygen in I'. PA1 X.sup.1 is nickel and/or cobalt, PA1 X.sup.2 is thallium, an alkali metal and/or an alkaline earth metal, PA1 X.sup.3 is phosphorus, arsenic, boron, antimony, tin, cerium, lead, niobium and/or tungsten, PA1 X.sup.4 is silicon, aluminum, titanium and/or zirconium, PA1 a is from 0.5 to 5, PA1 b is from 0.01 to 3, PA1 c is from 3 to 10, PA1 d is from 0.02 to 2, PA1 e is from 0 to 5, PA1 f is from 0 to 10 and PA1 n is a number which is determined by the valency and frequency of the elements differing from oxygen in II. PA1 the fact that the coat thickness can be varied, PA1 a high adhesive strength of the oxidic active material in combination with a completely satisfactory specific surface area thereof, PA1 greater homogeneity of the resulting coat thickness both over the surface of one carrier and over the surface of different carriers and PA1 satisfactory output of the production process.
This applies in particular in the case of hollow cylindrical carriers whose rolling behavior, in contrast to carrier spheres, has a preferred direction and is responsible for the prior art processes for the preparation of coated catalysts having catalytically active oxide materials being essentially limited to spherical coated catalysts.
DE-A 20 25 430 discloses that coated catalysts based on catalytically active oxide materials can be prepared by applying the catalytically active material to the carrier by the plasma spray or flame spray method. The disadvantage with regard to the suitability of this method is that at least one main component At must be fusible at the working temperature of the flame spray or plasma burner. Another disadvantage of this method is that the size of the specific catalytically active surface area is as a rule unsatisfactory. All embodiments of DE-A 20 25 430 are spherical coated catalysts. As a comparative example, DE-A 20 25 430 includes a process for the preparation of spherical coated catalysts, in which an aqueous solution containing oxalic acid and the catalytically active oxide material in dissolved form is sprayed onto hot carrier spheres. The disadvantage of this procedure is that it can be used only in the case of catalytically active oxide materials which are soluble in water. In addition, it leads to irregular coat thickness and unsatisfactory coat adhesion owing to the abrupt evaporation of the solvent at the surface of the hot carrier sphere. This procedure, too, results in considerable losses of active material.
DE-A 16 42 921 relates to the preparation of spherical oxidic coated catalysts by spraying a liquid containing the oxidic active material in dissolved or suspended form onto hot spherical carriers. DE-A 16 42 921 recommends water or an organic solvent, such as alcohol or formamide, as a solvent or suspending medium. Here too, the disadvantages include the fact that the water or solvent is evaporated virtually all at once as soon as the sprayed material comes into contact with the hot carrier, thus reducing the adhesive strength of the coat.
DE-A 25 10 994 corresponds essentially to DE-A 16 42 921, except that it includes annular carriers. It is also limited to catalytically active oxide materials which consist essentially of a vanadium/titanium mixed oxide.
DE-A 21 06 796 discloses the preparation of coated catalysts by spraying aqueous suspensions of the catalytically active oxidic material onto the agitated carriers. This procedure has the same disadvantages as described above for the spraying on of aqueous solutions containing the oxidic active materials in dissolved form. This applies in particular to spraying onto heated carriers. Even the recommended concomitant use of an aqueous polymer emulsion as a binder cannot remedy these disadvantages; rather, the presence of a polymer emulsion complicates the coating operation through film formation processes which are difficult to control. Although German Published Application DE-AS 21 06 796 also mentions cylinders as useful carriers, the embodiments do not include any corresponding-examples.
DE-A 26 26 887 attempts to reduce the disadvantages of DE-A 21 06 796 by carrying out spraying of the aqueous suspension onto carrier spheres which are only at from 25 to 80.degree. C. According to DE-A 29 09 671, page 5, column 10, this procedure may however result in the sprayed carrier elements sticking together. In order to increase the adhesive strength of the oxidic catalytically active coat to the surface of the carrier, DE-A 26 26 887 recommends the incorporation of inorganic hydroxy salts in the aqueous suspension to be sprayed on, which salts hydrolyze in aqueous solution to hydroxides and, after completion of the coated catalyst, form catalytically inert components of catalytically active oxide materials. However, the disadvantage of this measure is that it requires dilution of the oxidic active material. DE-A 26 26 887 also mentions rings and cylinders as possible carriers, but the examples as a whole are limited to spherical coated catalysts.
DE-A 29 09 670 corresponds essentially to DE-A 26 26 887. According to the description of DE-A 29 09 670, mixtures of water and alcohol may also be used as the suspending medium. After the suspension of the catalytically active oxide material has been sprayed on, the moisture content is eliminated by passing over hot air. The embodiments of DE-A 29 09 670 also include annular coated catalysts. However, only water is used as the suspending medium in all embodiments. The disadvantage of the procedure of DE-A 29 09 670 is the tendency of the sprayed moldings to agglomerate, as already mentioned with reference to DE-A 26 26 887. Furthermore, in the case of the annular coated catalysts, the resulting specific, catalytically active surface area of the coat of oxide material is generally unsatisfactory.
GB-1 331 423 relates to a process for the preparation of spherical oxidic coated catalysts, wherein an aqueous suspension or solution is formed from catalyst precursors and an organic assistant whose boiling point at atmospheric pressure is at least 150.degree. C. and which is soluble in water, carriers are added to said suspension or solution and the liquid components are removed by evaporation with occasional stirring. The coated carriers thus obtained are then calcined and the catalyst precursor layer is converted into active oxide. The disadvantage of this procedure is that the resulting coated catalysts have relatively irregular coat thickness. Furthermore; the adhesion of the coat to the carrier surface is unsatisfactory since the calcination of the catalyst precursor material generally releases in an uncontrolled manner gaseous compounds which cause loosening of the structure.
EP-A 286 448 and EP-A 37 492 recommend the preparation of coated catalysts by the spray process described above or by the process of GB-1 331 423, with the disadvantages stated above.
EP-B 293 859 discloses a process for the preparation of spherical coated catalysts by using a centrifugal-flow coating apparatus. This procedure leads to particularly uniformly worked coat thicknesses, both over the individual sphere surface and over different sphere surfaces. However, the disadvantage of the procedure of EP-B 293 859 is that it recommends coating the carrier spheres not directly with the catalytically active oxide material but with a precursor material thereof. The latter is converted into the former by subsequent combustion (calcination) at elevated temperatures (a few hundred degrees Celsius). As a rule, the calcination is accompanied by spontaneous, ie. more or less uncontrolled, thermal decompositions of components present in the precursor material to give gaseous products which, on the one hand, result in the formation of special pore distributions and a large specific catalytically active surface area (up to 15 m.sup.2 /g) but on the other hand reduce the adhesion of the catalytically active coat to the surface of the carrier. Calcination after coating is also disadvantageous in that spoiled batches may be produced during calcination (for example in the case of an incorrect calcination atmosphere). Working up thereof is substantially more complicated when coating has already been carried out. EP-B 293 859 mentions water, alcohol and acetone as binders, in addition to ammonium nitrate, graphite and starch.
DE-A 25 26 238, U.S. Pat. No. 3 956 377 and DE-A -2351 151 disclose a process for the preparation of spherical oxide coated catalysts, in which the carrier spheres are first moistened with water or another liquid, such as Petroleum ether, as a binder. The catalytically active oxide material is then applied to the binder-moistened carrier by rolling the moist carrier in the pulverulent catalytically active oxide material. The disadvantage of this procedure is that the achievable coat thickness is limited by the binder absorptivity of the carrier, since the binding of the total pulverulent oxide material to be taken up is dependent on this amount of binder taken up by the carrier. A further disadvantage of the method is that the degree of moistening of the particular surface layer during the coating process varies continuously, ie. the base layer comes into contact with the moisture of the uncoated carrier. The moisture then has to migrate initially through the base layer to the surface thereof in order to be able to bind further active material, etc. Consequently, an onion-like shell structure is obtained, the adhesion of successive layers to one another being particularly unsatisfactory. As a rule, the application of pressure causes the individual layers to peel away one after another. In all embodiments, the sole binder used is water.
DE-A 29 09 671 attempts to reduce the disadvantages of the procedure described there by introducing the spherical carriers into an inclined rotating turntable. The rotating turntable passes the spherical carriers periodically through two metering apparatuses arranged one after the other at a certain distance. The first of the two metering apparatuses corresponds to a nozzle through which the carrier spheres are sprayed with water and moistened in a controlled manner. The second metering apparatus is located outside the atomization cone of the water sprayed in and serves for feeding in the finely divided oxidic active material (for example via an oscillating conveyor). The carrier spheres moistened in a controlled manner take up the added catalyst powder, which is compacted to a cohesive coat on the outer surface of the carrier spheres as a result of the rolling movement. The carrier sphere provided in this manner with the base coat passes as, so to speak, fresh carrier once again through the spray nozzle in the course of the subsequent rotation, is moistened in the same controlled manner to enable it to take up a further layer of finely divided oxidic active material in the course of further movement, etc. The coat thickness can be adjusted essentially in a controlled manner by the method described. Furthermore, the homogeneity of the coat structure is improved. By passing in hot air, the water used as the binder can be finally removed. A further advantage of the procedured described is that the added finely divided oxidic active material can be such that it is completely taken up during the coating so that no losses of active material occur. However, a disadvantage of the procedure described is that the sole use of water as the binder does not produce completely satisfactory adhesion of the coat to the surface of the carrier sphere. In addition, the specific active surface of the resulting oxidic active material coat is in general not completely satisfactory.